1. Field of the Invention
The present invention relates to porous materials, and more specifically, to porous materials comprising silica or similar metal oxides, which have superior water vapor adsorption properties and, in particular, superior water vapor adsorption properties at low vapor pressures.
2. Description of the Related Art
Fine pore silica porous materials having pore diameters of between 1.5 nm-30 nm have been synthesized. For example, J. Am. Chem. Soc., 114, 10834 (1992), U.S. Pat. No. 5,256,277, and U.S. Pat. No. 5,334,368 describe methods for synthesizing silica porous materials using surfactants and a silica sol. Furthermore, Bull. Chem. Soc. Japan, 69, 1449 (1996) describes methods for making silica porous materials using surfactants and layered silicates.
However, in the method described in J. Am. Chem. Soc., 114, 10834 (1992), the surfactants form micelles and the synthesis proceeds using the micelles as a template. Therefore, it is difficult to synthesize silica porous materials having small, fine pores by using octyltrimethylammonium halides and decyltrimethylammonium halides as surfactants, because these surfactants cannot readily form micelles. In addition, in the method described in Chem. Mater., 11, 1110 (1999), synthesis is performed using a surfactant having a concentration that is greater than the critical micelle concentration. Therefore, porous materials with fine pores cannot be obtained, in particular, when octyltrimethylammonium halides are used. Consequently, there is a long felt, but as yet unsatisfied, need for porous materials having small, fine pores that provide high water vapor adsorption capacity at low relative vapor pressures.
Therefore, one object of the present teachings is to provide improved porous materials. For example, in one aspect of the present teachings, porous materials having fine pores are taught. Further, methods for making such porous materials are also taught.
In another aspect of the present teachings, porous materials having various water vapor adsorption capacities are taught. For example, the following porous materials are taught, which have useful water vapor adsorption capacities defined by the amount of adsorbed water vapor at a specific relative vapor pressure in a water vapor adsorption isotherm:
(1) Less than or equal to 0.1 g/g at a relative vapor pressure of 10%, and greater than or equal to 0.2 g/g at a relative vapor pressure of 28%.
(2) Less than or equal to 0.1 g/g at a relative vapor pressure of 10%, and greater than or equal to 0.25 g/g at a relative vapor pressure of 28%.
(3) Less than or equal to 0.1 g/g at a relative vapor pressure of 20%, and greater than or equal to 0.35 g/g at a relative vapor pressure of 35%.
(4) Less than or equal to 0.1 g/g at a relative vapor pressure of 25%, and greater than or equal to 0.4 g/g at a relative vapor pressure of 40%.
(5) Less than or equal to 0.1 g/g at a relative vapor pressure of 30%, and greater than or equal to 0.48 g/g at a relative vapor pressure of 50%.
(6) Less than or equal to 0.15 g/g at a relative vapor pressure of 40%, and greater than or equal to 0.60 g/g at a relative vapor pressure of 60%.
(7) The difference in the amount of adsorbed water vapor between any two points within a range of relative vapor pressure from greater than or equal to 10% to less than or equal to 28% is greater than or equal to 0.16 g/g, preferably, greater than or equal to 0.18 g/g.
(8) Less than or equal to 0.1 g/g at a relative vapor pressure of 10%, and greater than or equal to 0.2 g/g at a relative vapor pressure of 25%.
(9) The difference in the amount of adsorbed water vapor between any two points within a range of relative vapor pressure from greater than or equal to 10% to less than or equal to 25% is greater than or equal to 0.12 g/g.
(10) Less than or equal to 0.1 g/g at a relative vapor pressure of 8%, and greater than or equal to 0.18 g/g at a relative vapor pressure of 18%.
(11) The difference in the amount of adsorbed water vapor between any two points within a range of relative vapor pressure from greater than or equal to 8% to less than or equal to 18% is greater than or equal to 0.12 g/g.
Throughout this specification, all water adsorption ratios are described in terms of grams of adsorbed water per gram of porous material.
Such porous materials may be preferably utilized, for example, to absorb water in air conditioning systems and more preferably, in air conditioning systems for use in vehicles.
In another aspect of the present teachings, methods for the forming porous materials are also taught. For example, representative methods include condensing a skeleton starting material in the presence of a surfactant and in a solution which has a concentration of the skeleton starting material of less than or equal to 0.4 mol/L and has a molar ratio of the surfactant to the skeleton starting material that is greater than or equal to 0.05 and less than or equal to 50.
Other representative methods include mixing a surfactant and a skeleton starting material in the presence of an aqueous solvent having a pH value of greater than or equal to 10 to form a liquid mixture, reducing the pH of the liquid mixture to a pH value of greater than or equal to 9, and removing the surfactant from a solid fraction that separates from the liquid mixture after reduction of the pH value of the liquid mixture.
These methods can provide porous materials having a uniform fine pore size distribution and a small pore size.
Other representative methods include condensing a skeleton starting material in a solution having a concentration of the skeleton starting material that is less than or equal to 0.4 mol/L and has a molar ratio of a surfactant to the skeleton starting material that is greater than or equal to 0.07 and less than or equal to 25. The surfactant may then be removed from the condensate and the condensate may be contacted with a solution comprising an acid, or with a salt of an acid, and a metal ion having a valence of greater than or equal to 3.
Other representative methods include condensing a skeleton starting material in a solution having a concentration of the skeleton starting material that is less than or equal to 0.4 mol/L and a molar ratio of a surfactant to the skeleton starting material that is greater than or equal to 0.07 and less than or equal to 25. Preferably, the skeleton starting material may contain silicon or aluminum as a metal element. The surfactant may then be removed from the condensate.
Because these representative methods include either (1) contacting the condensate with a solution comprising an acid, or a salt of an acid, and a metal with a valence of greater than or equal to 3, or (2) Si and Al as a metal element in the skeleton starting material, it is possible to obtain porous materials with good moisture resistance.